1. Field of the Invention
The present invention relates to a method and apparatus for the manufacture of highly pure terephthalic acid with closely controlled output quality with respect to amount and levels of contaminant content, namely 4-carboxy-benzaldehyde (4-CBA) and optical density which affects coloration of the terephthalic acid (TA).
2. Description of the Prior Art
Heretofore it has been known to manufacture TA by first preparing a feed including a p-dialkylbenzene such as para-xylene mixed with a catalyst such as cobalt and/or manganese and a bromine from a bromine source plus a solvent system. Then the feed preparation is fed to one or more reactors with air and a controlled exothermic oxidation reaction takes place in the reactor(s) to form TA.
Then a TA slurry from the reactor(s) is supplied to a first crystallizer of a crystallization stage which may include one or more crystallizers. Again, air is supplied to the first crystallizer along with the TA slurry and some further oxidation occurs. The TA is then supplied to a product recovery stage where it is separated and dried. Dry TA produced is then sent to storage.
In such a method for the manufacture of TA, it has been proposed to provide certain controls to enhance the purity or quality of the TA output product produced.
In this respect, reference is made to Japanese Pat. No. 160330/79 for: METHOD FOR THE MANUFACTURE OF HIGHLY PURE TEREPHTHALIC ACID WITH CONSTANT QUALITY issued to the Kuraray Yuka Co., Ltd.
In this Japanese patent, it is taught to control the reaction temperature, the supply rate of the TA p-dialkylbenzene, the catalyst concentration, the supply rate of air and/or water content in the acetic acid solvent supplied to the feed to maintain constant the concentration of H.sub.2 or CH.sub.4 in the exhaust gas or the generation of H.sub.2 or CH.sub.4 and the ratio of the gaseous carbon components generated and the supply rate of the p-dialkylbenzene to the TA production process.
Stated otherwise, the variability of crude optical properties and 4-CBA are controlled by keeping both the H.sub.2 (or CH.sub.4) and CO.sub.x production rates constant in a reactor.
Secondly, the H.sub.2 and CO.sub.X production rates are maintained constant by manipulating one or more of the following variables or parameters: reaction temperature, p-xylene feed rate, air feed rate, feed mix composition and solvent water content.
Thirdly the method disclosed in the Japanese patent teaches that variability for both 4-CBA and optical density were reduced from 6 to 13% to 2 to 3% of the mean value.
As will be described in greater detail hereinafter, the method of the present invention and the apparatus implementing same differ from the teachings of the Japanese patent by providing for the continous measurement of indicator variables for both controlling optical density and 4-CBA content in the TA output product as opposed to utilizing discrete gas samples to monitor the optical density or opticals of the output product.
Secondly, the method and apparatus of the present invention allow for control of the level of the optical density (opticals) and the 4-CBA content in addition to reducing variability as opposed to the teachings of the Japanese patent which only teach reducing variability.
The method of the present invention is carried out on the basis of the recognition of the relationship between product 4-CBA content levels, colors or opticals of the TA product, and reactor vent oxygen concentration. In this respect, the following correlation was noted: ##EQU1##
With this relationship, the method of the present invention is utilized to control variability and level of crude TA opticals indirectly by controlling the reactor 4-CBA production and the reactor vent oxygen concentration. Typically, reactor and product 4-CBA levels are continuously monitored in the process by using two indicator variables, namely the reactor carbon dioxide production and the secondary oxidation oxygen uptake in the first crystallizer for controlling 4-CBA content. Then, in addition, reactor vent oxygen concentration is monitored for controlling optical density of the TA output product.